Mercator hoc monile carius vendit quam illam armillam.

Because carius is an adverb, while carior is an adjective.

Here, carius modifies the verb vendit (sells), so it means more expensively or at a higher price. The sentence is talking about how the merchant sells the necklace, not describing the necklace itself as dearer.

• carior = dearer / more expensive (adjective)
• carius = more dearly / more expensively (adverb)

So Latin uses:

• Mercator hoc monile carius vendit = The merchant sells this necklace more expensively

Because they are different genders.

• monile is a neuter noun
• armilla is a feminine noun

The demonstratives must agree with the nouns they go with in gender, number, and case:

• hoc monile = this necklace
  • hoc is neuter singular accusative
  • monile is neuter singular accusative
• illam armillam = that bracelet
  • illam is feminine singular accusative
  • armillam is feminine singular accusative

So the forms differ because the nouns they modify belong to different genders.

Because they are the direct objects of the verb vendit.

The merchant is selling what?

• hoc monile = this necklace
• and, in the comparison, illam armillam = that bracelet

In Latin, the direct object usually goes in the accusative case. That is why both noun phrases are accusative.

Quam means than in comparisons.

The sentence is comparing two acts of selling:

• the merchant sells this necklace
• more expensively than he sells that bracelet

Latin often leaves out words that are understood. So the full idea is:

• Mercator hoc monile carius vendit quam illam armillam (vendit).

The second vendit is omitted because it is obvious from context.

Because this is not the kind of comparison that uses the ablative of comparison.

With comparative adjectives, Latin can sometimes compare in two ways:

• quam + same case
• or the ablative without quam

For example:

• Marcus altior quam Gaius est
• Marcus Gaio altior est

But here the comparison is built around the adverb carius, and illam armillam is really the object of an understood vendit. So Latin uses quam and keeps illam armillam in the accusative.

In other words, the structure is basically:

• He sells this necklace more expensively than he sells that bracelet.

Vendit is:

• 3rd person singular
• present tense
• active voice
• indicative mood

It comes from vendo, vendere, vendidi, venditum = to sell

So vendit means:

• he sells
• or sometimes he is selling, depending on context

Here it means the merchant sells.

Mercator is nominative singular, so it is the subject of the sentence.

It comes from mercator, mercatoris = merchant

Since vendit is he/she sells, we look for a singular nominative noun to serve as the subject. That noun is mercator.

So:

• mercator = the merchant → subject
• vendit = sells

Because Latin word order is much more flexible than English word order.

English depends heavily on position:

• The merchant sells this necklace more expensively than that bracelet.

Latin depends much more on endings than on position. The endings show which word is the subject, which words are objects, and which word modifies which.

So Mercator hoc monile carius vendit quam illam armillam is understandable because:

• mercator is nominative → subject
• hoc monile is accusative → object
• carius modifies vendit
• illam armillam is accusative after quam, with an understood vendit

The chosen order may help emphasize the comparison or simply sound natural in Latin prose.

Not exactly by themselves in this sentence. Here they are demonstrative adjectives, because they modify nouns:

• hoc monile = this necklace
• illam armillam = that bracelet

Their dictionary forms are:

• hic, haec, hoc = this
• ille, illa, illud = that

They can also be used as pronouns, but here they are attached to nouns and agree with them.

No. Monile is a third-declension neuter noun.

Its dictionary form is:

• monile, monilis = necklace

That is why its accusative singular is monile, not something like monilum.

A learner might expect a neuter direct object to end in -um, but that is only true for many second-declension neuters. Third-declension neuters often have different patterns.

Because armilla is a first-declension feminine noun, and -am is the normal accusative singular ending for that declension.

Its dictionary form is:

• armilla, armillae = bracelet / armlet

So:

• nominative singular: armilla
• accusative singular: armillam

Since it is the direct object of the understood verb after quam, the accusative armillam is required.

Yes. A fuller version would be:

• Mercator hoc monile carius vendit quam illam armillam vendit.

That literally means:

• The merchant sells this necklace more expensively than he sells that bracelet.

Latin often omits repeated words when they are easy to understand, so the shorter original sentence is perfectly normal.